So not sure if people have speculated on this or
not yet...

https://www.google.com/search?q=sylveon&hl=en&client=safari&tbo=d&source=lnms&tbm=isch&sa=X&ei=cWIeUdb4Bcjo0gHl2IHgCg&ved=0CAgQ_AUoAQ&biw=480&bih=268#i=2If you look at the chart every eeveelution is corresponding to one that its either strong/weak against (leafeon/vaporeon espeon/umbreon glaceon/flareon) and respectively jolteon corresponds with sylveon. Which leads me to believe which ever type it is will either be strong against electric (ground) or weak against electric (flying) or some new type thats either strong/weak against electric. I extremely doubt sylveon is ground though.